1. Field of the Invention
The present invention relates to an ink jet recording apparatus which can control a dew point of air in the apparatus.
2. Description of the Related Art
An ink jet recording apparatus performs printing by ejecting ink of tens pico liter through many nozzle holes each having tens μm diameter onto a recording medium such as paper. An ink ejecting part comprises a nozzle plate in which many nozzle holes are provided, a pressure chamber communicating with each nozzle hole, a common liquid chamber for supplying the ink to the pressure chambers, and a unit that generates pressure in the pressure chamber.
There are two types of apparatuses that generate pressure in the pressure chamber. One of them is a type in which air bubbles are generated in the pressure chamber by Joule heat, and the other is a piezoelectric type in which the pressure chamber is deformed by a piezoelectric element. In the piezoelectric type, since the amount of ink to be ejected and ink ejection speed are more easily controlled than in the type that employs air bubbles, it is expected that exacter printing is possible.
In the piezoelectric type, in order to realize fine printing, it is necessary to increase the in-plane density of the nozzle holes, the pressure chamber, and the piezoelectric element set on the pressure chamber. Therefore, it is necessary to reduce the area of the piezoelectric element. In order to form the piezoelectric element having the predetermined area, a method is used, which comprises steps of: firstly, forming a piezoelectric film on the whole of a base material; and thereafter, forming a resist pattern by photolithography to remove the piezoelectric film of no-resist portion by etching. By this method, it is impossible to make the area of the piezoelectric element smaller than the thickness of the piezoelectric film. Therefore, in order to form a piezoelectric element having a smaller area, it is necessary to use a thinner piezoelectric film.
For the piezoelectric element used in the ink jet recording apparatus, it is necessary to have a high piezoelectric constant. As its material, lead titanate oxide (PT); lead titanate zirconium oxide (PZT); and magnesium additive, manganese additive, cobalt additive, iron additive, nickel additive, niobium additive, scandium additive, tantalum additive, and bismuth additive to PZT have been generally known. In order to generate the pressure in the pressure chamber, it is generally necessary to apply an electric field of several KV/cm or more to the piezoelectric element thereby to give strain to the element.
It has been known that many defects such as minute cracks and pores exist in the piezoelectric element. Under existence of moisture, generally, in a case where a high electric filed is applied to the piezoelectric element that includes lead, large electric current flows to the lead compound at the defect part and its surroundings, and their portions are broken by the Joule heat, so that a large hole can be formed.
In order to prevent formation of the hole due to the Joule heat, for example, two methods are known. A first method is to thicken the piezoelectric element. In a case where the piezoelectric element is thin, a large defect passing through the element is produced by a break, so that such disadvantage is produced that an upper electrode and a lower electrode can short electrically, or the displacement property changes. On the contrary, in a case where the element has the thickness of some degree, even if the defect breaks, such a hole passing through the element cannot be made, so that a large influence is given on the piezoelectric property. A second method is to seal the piezoelectric element and a desiccant in a container in order to remove the moisture. For example, this method is proposed in JP-A-4-349675.
However, according to the first method, in the case where the thickness of the piezoelectric element is made large, the break is not caused even under a high humidity. However, a high voltage must be applied in order to make the displacement large, which increases power consumption. Further, if the film thickness is large, it becomes difficult to increase the in-plane density of the element.
According to the second method, if the piezoelectric element is sealed, the sealing work must be performed in a low humidity environment where little moisture is present, which requires much labor in the case of mass production in a factory and increases the manufacturing cost. Further, since the piezoelectric element is covered with a box in order to seal the element, entry of moisture from a contact surface between the box and the element must be strictly prevented, which requires much labor and similarly increases the manufacturing cost.